I'm running 3 filters total, 2x Eheim canister with combined FL of 1350l/hr, inlets at rear corners, outlet using spray bars. Also, there's an OHF rated at 600l/hr.
As with media, I use what comes with the Eheims, ceramic rings and coral chips inside the OHF underneath a coarse and then a fine filter wool.

Feeding: 1-2 times per day, alternate with FBW and presoaked pellets, to be consumed under 3-4 mins.

Issue: pH stays between 6.2-6.6, causing slight cloudy eyes to the larger fish and occasional rubbing. Also, low pH encourages growth of harmful bacteria, evidently on the walls of the tank. I've read that established bio filtration is prone to release a lot a acids into the water. The last time I serviced the canisters were 2 months and 3 months ago, respectively. I do them in separate months so as not to cause my nitro cycle to crash.
So, what are my options? Remove one canister to reduce bio filtration and hence acid production? I don't think my bio load is that high considering 30L/fish. And I'd prefer not having to do daily 25-30% WC to maintain pH between 6.8-7.2 range. All other water parameters are good.

Additional info:
I'm aware the use of coral chips can help buffer water pH.
I put a handful in each of the canisters, as well as inside the OHF. I'm considering to drop a small bag ~500g inside the tank but have not done so. First, it is unsightly. Second, I have heard horror stories where the CC continued to dissolve when fully submerged in tank water, sending the pH to 7.8-8 range, causing pH burn to the fish. Any ideas from your prior experience?

However, there is one point that leads to my believe. Do touch your Eheim filter box, is it slightly warm?
For my case, it is. When there is an increase of temperature, oxygen reduced.
Hence inside the filter box (pack volume), the temperature would even be higher = to much lower oxygen.
Good bacteria needs a lot of oxygen.

Again in my old threads, for goldfish, i recommend trickle box filtration.
Where there is lots of oxygen.
It is a proven theory, aeration of the water reduced ammonia.
(What media to put in the trickle box is important too)

Trickle filtration system works great but it is unpleasant to look at.
In any case, I serviced both canisters yesterday and dropped a bag of 500g CC in the tank. They seem well and lively this AM. Will test the water parameters this afternoon.
Thanks for your advice!

I have done more research on types of filtration.
Indeed canisters are not the best for GF.
For bio filtration to work best, i.e. High BB activity, high oxygen level is required. This can only be achieved through rapid air-water interface or agitation. Sounds like trickle system to me.
In any case, I'd have to stick to more regular maintenance of the canister + WC.